1. Field of the Invention
The present invention relates to the field of communications. More particularly, the present invention relates to monitoring traffic on a repeater used in a wireless communication system, such as a framed shared channel wireless communication system.
2. Description of the Related Art
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. A typical wireless data system, or network, provides multiple users access to one or more shared resources. A system may use a variety of multiple access techniques such as frequency division multiplexing (FDM), time division multiplexing (TDM), code division multiplexing (CDM), and others. Examples of wireless networks include cellular-based data systems. The following are several such examples: (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard), and (4) the high data rate (HDR) system that conforms to the TIA/EIA/IS-856 standard (the IS-856 standard).
Repeaters are used in wireless communication systems in order to extend the range and coverage of the communication system. In general, repeaters receive and retransmit signals at the physical layer, and are able to provide satisfactory operation regardless of the standard being used by the wireless communication system. Repeaters are advantageous in that they provide an economical means to extend the range of a framed shared channel wireless communication system, particularly in cases where sufficient capacity exists, but signal propagation is difficult.
One technique for taking power measurements relevant to traffic is obtaining rise-over-thermal (RoT) measurements. In a communication system such as a CDMA system, RoT is a signal property value which is useful for providing an indication of the channel loading on the reverse link. The RoT value is the ratio, typically given in decibels (dB), of total power received from all users at a receiver, over the thermal noise. Based on theoretical capacity calculations for a reverse link, there is a theoretical curve that shows the rise-over-thermal value increasing with loading. The loading at which the rise-over-thermal value is infinite is often referred to as the “pole”. In a typical CDMA system, a loading that has a rise-over-thermal value of 3 dB corresponds to a loading of about 50%, or about half of the number of users that can be supported when at the pole. As the number of users increases and as the data rates of the users increase, the loading becomes higher. Correspondingly, as the loading increases, the amount of power that a remote terminal must transmit increases. Similar considerations exist for other types of communication systems. The rise-over-thermal value and channel loading are described in further detail by A. J. Viterbi in “CDMA: Principles of Spread Spectrum Communication,” Addison-Wesley Wireless Communications Series, May 1995, ISBN: 0201633744. The Viterbi reference provides classical equations that show the relationship between the rise-over-thermal value, the number of users, and the data rates of the users.
RoT is generally referenced to the input power of the receiver with no traffic. It is therefore possible to take a measurement of the equivalent thermal noise floor, with the increase in output power described as the rise. Rise-over-thermal (RoT) measurements are used to estimate load of a wireless receiver, and therefore can be used to measure repeater load. RoT is a ratio of thermal and the total received input power. The key assumption is that one can derive a reasonable model for the aggregate impact of all users by starting with the impact of a single, average user. RoT can be derived from:
                              Z          l                =                                            ∑                              i                =                1                            N                        ⁢                                          P                i                            ⁢                              g                i                                                                        N              0                        ⁢            W                                              Equation        ⁢                                  ⁢                  (          1          )                    where:    Z1 is the RoT for a communication station,    Pi is the transmitted power for the user I,    gi is the gain for the user,    N is the number of users,    N0 is the receiver's thermal noise density,    W is the receiver bandwidth, given in Hz
In some cases, repeaters are in locations where link traffic volume is not a significant issue; however, there are some cases in which the repeater is used in a circumstance in which link traffic and network capacity are considerations. As a result, there are cases in which it is desired to measure link traffic on repeaters. Specifically, it is desirable to include the ability to estimate repeater traffic load based on measurements of repeater reverse-link output power.
Reverse-Link Loading at the Base Station
In the case of the reverse link, an important parameter is the RoT, which corresponds to the reverse link loading. A loaded CDMA system attempts to maintain the RoT such that the system operates at or below a critical level of RoT. The critical level of RoT occurs when the cell shrinks and quality of service (QoS) starts to degrade. If the RoT is too great, the range of the cell is reduced and the reverse link is less stable. A large RoT also causes small changes in instantaneous loading that result in large excursions in the output power of the mobile station. A low RoT can indicate that the reverse link is not heavily loaded, thus indicating excess capacity. It will be understood by those skilled in the art that methods other than measuring the RoT that can be used to determine the loading of communication devices.
Assume an average target Eb/Nt is required by each call in a sector in order that all calls meet the desired frame-error-rate (FER). Call this target value T:
                    T        =                              E            b                                N            t                                              Equation        ⁢                                  ⁢                  (          2          )                                    where        Eb is the average energy per data bit at the base station receiver.        Nt is the sum of the base station receiver's thermal noise density N0 and the interference power density I0.        I0. is determined by:        
                              I          0                =                                            (                              n                -                1                            )                        ⁢            C                    W                                    Equation        ⁢                                  ⁢                  (                      2            ⁢            a                    )                    
T is the ratio of the average power per user received at the base station, νC, to the average data rate νR:
                              E          b                =                              υ            ⁢                                                  ⁢            C                                υ            ⁢                                                  ⁢            R                                              Equation        ⁢                                  ⁢                  (          3          )                                    where        ν is the average voice activity factor,        C is the average power received per full-rate user,        R is the data rate,        N0 is the receiver's thermal noise density.        
If there are n total users in the sector, the interference power density from the other users is:(n−1)C/W  Equation (4)                where W is the signal bandwidth.        
To account for the voice activity factor, one can multiply this quantity by ν (typically taken to be 0.4). To account for other cell interference, divide this quantity by F, the ratio of in-cell to total interference power density (typically taken to be 0.65).
Substituting these values into Equation (2), we obtain
                    T        =                              C            R                                              N              0                        +                                                            (                                      n                    -                    1                                    )                                ⁢                C                ⁢                                                                  ⁢                υ                            WF                                                          Equation        ⁢                                  ⁢                  (          5          )                    
This equation can be solved for C. Defining W/R=g as the processing gain, and approximating n−1 as n (which the receiver AGC does anyway), one obtains an expression for the average power received per full-rate user at the base station:
                    C        =                              (                                          N                0                            ⁢              W                        )                    ⁢                      (                          T              g                        )                    ⁢                      1                          1              -                              n                ⁡                                  (                                                            υ                      ⁢                                                                                          ⁢                      T                                        gF                                    )                                                                                        Equation        ⁢                                  ⁢                  (          6          )                    
If the base station is also servicing a repeater, this received power per user is the same whether the call comes through the repeater or not. In fact the base station generally can't differentiate between direct connections to users and connections through repeaters.
Reverse-Link Power Out of the Repeater
The equivalent thermal noise floor of the repeater can be represented by:Γ=k(T0+Te)WGR  Equation (7)                where:        GR is the repeater's reverse-link gain,        Then the power being transmitted out of the repeater on the reverse link is given by        
                              P          R                =                  Γ          +                                                    n                r                            ⁢              C                                                      FL                p                            ⁢                              G                d                            ⁢                              G                a                                                                        Equation        ⁢                                  ⁢                  (          8          )                                     where:        Ga is the base station antenna gain,        Gd is the repeater's donor antenna gain,        LP is the path loss between base station and repeater,        nr is the number of calls passing through the repeater, where        F is the ratio of in-cell to total interference power density        
Dividing both sides of this expression by the repeater thermal noise floor gives the repeater output power expressed as RoT.
Equation (8) is of interest in that it provides an indication of receiver capacity for users as a function of carrier output power, the number of users, number of users on multiple carriers, percentage of base transmitter (BTS) capacity, and capacity in Erlangs of traffic.
U.S. Pat. No. 6,469,984, commonly assigned, describes a method for estimating the reverse link traffic going through the repeater by measuring the rise-over-thermal value in the repeater reverse link channel by monitoring traffic on a CDMA repeater. A measuring circuit receives a metric related to CDMA repeater operations and determines the amount of call traffic on the CDMA repeater based upon the metric. Determining that the traffic over the CDMA repeater is too heavy is used to indicate the need to replace the CDMA repeater with the base station to provide better wireless communication service. The circuit monitors the amount of the call traffic based on signal power of the CDMA repeater and a power meter reads the signal power of the CDMA repeater so that the circuit receives the signal power of the CDMA repeater from the power meter. The circuit then determines the number of users on the CDMA repeater based on the signal power from the power meter. This determination is made by a formula where the signal power is proportional to the number of users on a CDMA repeater. The repeater measurement system may store the number of repeater users in memory or transmit the number of repeater users to a remote location.
In practice, the actual rise-over-thermal value is a small number of dB, and the gain uncertainty of the reverse link amplifier gain and noise figure make the measurement uncertainty approximately that of the desired rise-over-thermal measurement. It would be desired to provide a way to calibrate the gain and noise figure of the repeater gain stage such that an accurate rise-over-thermal measurement can be made.
One of the problems in measuring equipment used in a wireless network is that it is difficult to take the equipment “off-line”. In the case of manual maintenance, it is possible to do so (take the equipment off-line) but there becomes a tradeoff between optimum time between testing and QoS. QoS is degraded by equipment frequently taking equipment off-line when users are connected to the equipment. In the case of framed shared channel wireless communication systems, each frame has a time period, that is either defined by the cognizant standard or that is otherwise predictable. For example, in the case CDMA wireless communication conforming to the IS-95 standard, the frame length is approximately 20 ms.